doc: document new TCP_KEEPCNT and TCP_KEEPINTVL socket option defaults

PR nodejs#32204 introduced new defaults for the TCP keep-alive socket options (see deps/uv/src/unix/tcp.c):
- "TCP_KEEPCNT" now defaults to 10 on all platforms
- "TCP_KEEPINTVL" now defaults to 1 on all platforms

Previously, "TCP_KEEPCNT" and "TCP_KEEPINTVL" were not set explicitly and OS-default values were used.

Fixes: nodejs#38298
Refs: nodejs#32204

doc/api/net.md

@@ -1006,6 +1006,10 @@ Set the encoding for the socket as a [Readable Stream][]. See
 ### `socket.setKeepAlive([enable][, initialDelay])`
 <!-- YAML
 added: v0.1.92
+changes:
+  - version: v12.17.0
+    pr-url: https://github.com/nodejs/node/pull/32204
+    description: New defaults for `TCP_KEEPCNT` and `TCP_KEEPINTVL` socket options were added.
 -->
 
 * `enable` {boolean} **Default:** `false`
@@ -1020,6 +1024,12 @@ data packet received and the first keepalive probe. Setting `0` for
 `initialDelay` will leave the value unchanged from the default
 (or previous) setting.
 
+Enabling the keep-alive functionality will set the following socket options:
+* `SO_KEEPALIVE=1`
+* `TCP_KEEPIDLE=initialDelay`
+* `TCP_KEEPCNT=10`
+* `TCP_KEEPINTVL=1`
+
 ### `socket.setNoDelay([noDelay])`
 <!-- YAML
 added: v0.1.90